1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet having both improved magnetic properties and properties of a glass film.
2. Description of the Prior Art
The grain-oriented electrical steel sheet is used as a core for transformers and other electrical machinery and apparatus. The magnetic properties required of the grain-oriented electrical steel sheet when used for the core are good excitation and watt loss.
The secondary recrystallization process by which grains having a (110) plane parallel to the rolling surface and an &lt;001&gt; axis in the rolling direction are developed, is utilized to produce the grain-oriented electrical steel sheet. The secondary recrystallized grains are referred to as the Goss texture. To develope the secondary recrystallized grains, a so-called inhibitor is used to inhibit the growth of primary recrystallized grains from occurring until the finishing annealing, more specifically until the stage at which the temperature is elevated to the annealing temperature for the secondary recrystallization. Known inhibitor include AlN, MnS, MnSe, and BN. At present a nitride inhibitor, such as AlN, a sulfide inhibitor, such as MnS, or both the nitride and sulfide inhibitors are mainly used. The inhibitor must be finely precipitated and dispersed in the steel, and must be neither dissolved nor varied in size up to a certain temperature region.
The starting material for producing the grain-oriented electrical steel sheet is Si-steel containing C and the inhibitor-forming elements. The Si content of the Si-steel is up to 4%. The Si-steel is first hot-rolled and then annealed if necessary, particularly when the AlN inhibitor is used. The hot-rolled strip is cold-rolled once or twice with an intermediate annealing. The cold-rolled strip having the final finishing thickness is decarburization-annealed and then subjected to the application of an annealing separator which is mainly composed of MgO. Then, the cold-rolled strip is finishing annealed. During the finishing annealing, the Goss texture is formed and, further, impurities such as N, S, etc. are removed from the steel into the glass film also formed during the finishing annealing. This glass film is an insulative film having a glass-like structure.
Recent strong trends toward energy conservation in the field of transformers and the like resulted in not only conventional studies of the inhibitor components but also studies of the glass film. Various proposals have been made with regard to the method for forming the glass film during the finishing annealing. For example, (a) Japanese Examined Patent Publication (Kokoku) No. 51-12451 describes a method for applying, to the sheet surface on which the SiO.sub.2 -containing insulating film is formed, the annealing separator which comprises, in addition to an Mg compound, from 2 to 40% of Ti compound; (b) Japanese Unexamined Patnet Publication (Kokai) No. 54-143718 describes an annealing separator which comprises mainly MgO, with the addition of an Sr-containing compound in an amount of from 0.1 to 10% in terms of metallic Sr, and, if necessary, a Ti compound in an amount of from 0.5 to 5% in terms of metallic Ti; and (c) Japanese Unexamined Patent Publication No. 58-107417 describes an annealing separator which comprises mainly MgO, and metallic Sb or an Sb compound in an amount of from 0.01 to 1.0%, the particle size of the Sb or Sb compound being 20 .mu.m or less when the content of the particles is 70% or more.
The annealing separator (a) above allegedly improves the adherence of the glass film to the steel sheet, enhances the electric resistance between the glass film and the steel sheet, and mitigates the embrittlement of the steel sheet.
The annealing separator (b) above allegedly eliminates the forstellite grains present directly beneath the steel sheet surface and moves the forstellite grains upwards into the glass film, due to the effects of Sr, thereby improving the adherence of the glass film to the steel sheet.
The annealing separator (c) above allegedly reduces, due to effect of Sb, the diameter of the secondary recrystallized grains without impairing the orientation alignment of the secondary recrystallized grains.